DESCRIPTION: The contribution of genetic factors to sleep disorders has been demonstrated in twin and family studies, and further understanding of the genetic underpinning of sleep disorders is considered an important area of research. Many of the suggested risk factors for sleep apnea (e.g., craniofacial morphology, obesity) have genetic determinants. However, a formal genetic analysis of these anatomical risk factors with sleep-recorded physiological measurements has not so far been conducted. The objective of the proposed study is to conduct such an analysis in a well-characterized U.S. population-based registry of elderly male twins who have been successfully followed for the past 30 years. Specifically, we propose: (1) to recruit a subsample of 150 pairs from the NAS-NRC World War II Twin Registry in which at least one of the twin brothers reported sleep apnea symptoms and a subsample of 30 control pairs in which both twins reported no symptoms; (2) monitor the subgroup of 180 twin pairs with overnight sleep recording; (3) collect on these subjects anthropometric measurements of weight, height, neck circumference, and craniofacial morphology; and (4) collect blood samples for determination of zygosity and DNA extraction for future molecular studies. The twin design is most powerful for estimating the genetic and/or environmental overlap between physiological measurements such as sleep-recorded disordered breathing, obesity, and craniofacial morphology. We have chosen to focus on an elderly male twin sample for which a wealth of data relevant to this study has been previously collected. Twin pair concordance or discordance for monitored sleep-disordered breathing will allow the full characterization of genetic/familial and individual environmental factors associated with the expression and severity of this condition. The investigators of this study represent a multidisciplinary team who have the knowledge and experience to conduct a comprehensive genetic study of sleep-disordered breathing in the only existing and well-studied U.S. twin registry of elderly men.